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Voltage-dependent alteration of enzyme substrate


, : Voltage-dependent alteration of enzyme substrate.

Voltage-sensing phosphatases (VSPs) consist of a voltage-sensor domain and a cytoplasmic region with remarkable sequence similarity to phosphatase and tensin homolog deleted on chromosome 1 (PTEN), a tumor suppressor phosphatase. VSPs dephosphorylate the 5? position of the inositol ring of both phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P3] and phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] upon voltage depolarization. However, it is unclear whether VSPs also have 3? phosphatase activity. To gain insights into this question, we performed in vitro assays of phosphatase activities of Ciona intestinalis VSP (Ci-VSP) and transmembrane phosphatase with tensin homology (TPTE) and PTEN homologous inositol lipid phosphatase (TPIP; one human ortholog of VSP) with radiolabeled PI(3,4,5)P3. TLC assay showed that the 3? phosphate of PI(3,4,5)P3 was not dephosphorylated, whereas that of phosphatidylinositol 3,4-bisphosphate [PI(3,4)P2] was removed by VSPs. Monitoring of PI(3,4)P2 levels with the pleckstrin homology (PH) domain from tandem PH domain-containing protein (TAPP1) fused with GFP (PHTAPP1-GFP) by confocal microscopy in amphibian oocytes showed an increase of fluorescence intensity during depolarization to mV, consistent with 5? phosphatase activity of VSP toward PI(3,4,5)P3. However, depolarization to 6 mV showed a transient increase of GFP fluorescence followed by a decrease, indicating that, after PI(3,4,5)P3 is dephosphorylated at the 5? position, PI(3,4)P2 is then dephosphorylated at the 3? position. These results suggest that substrate specificity of the VSP changes with membrane potential.

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